While I do turn mine on for a few minutes once a week or so in the winter, I have never just left it on. The additional wear and tear on the compressor for no good reason is mostly why, and I can feel the power drain on the engine from the AC and look forward to winter when I can turn it off. I have never had my AC refuse to work at the beginning of AC season. But, to each his own.

Regarding the pressure switches, on the 124 there are two. The one for the compressor is the one where the wires plug directly onto the switch. The switch with pigtails is the one for the AUX fans. Perhaps the 210 is the same.

The pods should be no factor. Jumper the pressure switch and see if the compressor engages.

Also, you heard a squeal. Be sure your serpentine belt is tight. If it is loose and lets the compressor slip, the AC system will shut down.

Regarding the pressure switches, on the 124 there are two. The one for the compressor is the one where the wires plug directly onto the switch. The switch with pigtails is the one for the AUX fans. Perhaps the 210 is the same.

The pods should be no factor. Jumper the pressure switch and see if the compressor engages.

Please, please, for the love of the W210, do NOT attempt to jumper any pressure switches! This car doesn't have a switch for that function.

Left-side warm is caused by a number of possible things. Left-side duovalve is certainly one of them. Low refrigerant level is another, and will be accompanied by a noticeable noise from the center vents.

If the left-side heater core temp sensor is bad, it could conceivably cause this as well, but I've not hear do that problem.

I have a theory about that. Since the right-side gets quite cold, clearly there is plenty of evaporation going on there. I don't think the problem would be the left side being full of vapor, and I wonder if it could be filling with oil. You know, that stuff that you need returned to the compressor once in a while.

The right side is colder than the left, I may need to add more freon. Since I don't have a set of 134 gauges I was being cautious.
As I added freon I got the "low freon" whistle from the expansion valve...then it went away. But, if different temps left and right is a symptom of low freon, I'll try a little more.
I usually believe in the worst case, so thought a sensor might be bad, telling the left it was cool when it wasn't.

You can find the high-side pressure from the pushbutton unit. Hold the REST button down for >5 seconds until the display changes, then use the AUTO buttons to scroll to entry #7. But the low-side pressure is very important as well; perhaps more so.

Matt L , I use the BIT (built in test) and got 5-6 on the pressure...sounds a little low, if that is the high side (90ish).

Thanks for the UK mercedes site...once I learn to speak English.. it will be another resource. Re-gas the system..(I used to have a couple of old BSA's , loved the instructions for overhaul..."tighten evenly and apply a final wrenching..." I still have some whitworth spanners to work on my MGB!